Biogas is a combustible gas obtained by fermentation, also referred to as methanisation, of organic waste of animal or plant origin in the absence of oxygen. It is mainly composed of methane and carbon dioxide. It also conventionally contains a little nitrogen, very little oxygen, water vapour, hydrogen sulphide (H2S) and numerous other substances in very small quantities, in particular volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), halogenated compounds, heavy metals and siloxanes. The nature and concentration of these substances vary in particular according to the methanisation method and the nature of the methanised material.
Biogas may be used as a fuel in replacement for natural gas. However, according to the uses sought, it may be necessary to purify the biogas in order to eliminate certain contaminating substances therefrom. For example, in the case of the use of biogas as an engine fuel, it is necessary to eliminate the siloxane compounds since, oxidised at high temperatures, siloxanes form deposits of silica that may seriously damage the equipment.
The purification of biogas is generally carried out by means of filters that capture the undesirable compounds through an adsorption phenomenon. When the material of the filters is saturated, the worn filters are replaced by new ones, with the economic and ecological consequences that result therefrom. Currently, the so-called regenerative solutions, that is to say methods in which the worn filters are regenerated and reused, are few in number.
The American patent application US 2013/0137567 presents a method for eliminating siloxanes in biogas by means of a regeneratable adsorbent material. The regeneratable material used is activated carbon in the form of granules disposed inside a tubular chamber with a rectangular cross section formed by two electrodes and two dielectric elements disposed respectively face to face. When the activated carbon is saturated with contaminant, it can be regenerated by passing an electric current between the two electrodes of the device. This technical solution does however pose a problem with regard to the maintenance of the electrical contact between the two electrodes: if the contact between the grains of activated carbon is too small, the electrical resistance of the regeneratable material becomes too high and the electric current no longer flows correctly. To remedy this problem, this American patent application US 2013/0137567 proposes installing a vibrating element intended to homogenise the contacts between the activated carbon granules. This solution is however not optimum: the presence of this vibrating element makes the device more complex, more expensive to manufacture, and difficult to reproduce on a large installation. Furthermore, even homogenised by means of the vibrating element, the regeneratable material consisting of activated carbon in the form of granules has high electrical resistance, which means that the electrical power necessary for obtaining sufficient regeneration is high.
One of the objectives of the present invention is to propose a novel regenerative method for eliminating siloxanes from a biogas that is more effective than those of the prior art.
Other regenerative methods for adsorbin, contaminants in gaseous streams have been described in the prior art. Mention can be made in particular of those described in the American patent application US 2006/0096454 and the European patent application EP 1 170 050. These documents describe the use of activated carbon in the form of a cloth as the adsorbent material able to be regenerated by applying an electric current. However, the contaminating compounds the adsorption of which is sought in these documents are essentially the residues of solvents such as toluene present in the ambient air, There is no question of the capture of siloxane compounds in a biogas. However, the adsorption and regeneration properties of an adsorbent material depend to a major extent on the nature of the compound adsorbed, its concentration and the nature and concentration of the other chemical species in the gas containing the contaminating compound. Furthermore, in these two documents, it should be noted that the regeneration step during which an electric current is applied in the adsorbent material is performed in the absence of a flushing stream.
There therefore remains the need to propose a simple and inexpensive method for eliminating siloxanes from a biogas and then regenerating the adsorbent material. Advantageously, it is wished for the adsorption of the siloxanes to be high but for the regeneration of the adsorbent material to consume as little energy as possible. In addition, it is wished for the adsorption capacity of the material to be maintained as high as possible during the adsorption/desorption cycles.